The tensioning of rod like members or studs can be done by applying a tightening torque to a threaded nut resting axially on the surface of the structure to be tightened. This tightening method does not allow controlling precisely the axial force transmitted to the rod, because of frictions between the nut in contact with the surface of the part to be tightened and between the nut and the rod. Moreover, this tightening method introduces torsion residual stresses into the rod, which results in using oversize rods. In addition, such a tightening generally leads to an important risk of damage of threads of the nut and of the rod cooperating together during tightening. The surface against which the nut is resting during its rotation at the time of tightening may also be damaged.
In order to avoid these disadvantages, a rod member or a stud may be axially tensioned before fitting-up a nut on the surface of a structure to be tightened. The nut blocks the rod when the axial force applied to the rod is released. The rod is thus pre-stressed only along its axis. An actuator can be used in order to transmit an axial traction force to the rod. Thus, after the nut is positioned on a threaded portion of the rod to be tensioned, the actuator is positioned so as to surround the head portion of the rod or stud and the nut. A traction force is applied in a longitudinal direction on a threaded extension of the head part of the rod member or stud and produces an elongation of the stud. This elongation facilitates the rotation of the tightening nut adapted to cooperate with a threaded portion of the stud so as to tighten the nut. The two mechanical parts can thus be secured together by the rod member or the stud without inducing a twist or a residual torsion stress to the stud.
Large mechanical assemblies must often be maintained by several threaded rods or studs. Multiple stud tensioning machines are then used to position, screw and unscrew as well as pre-tension a plurality of threaded studs. Examples are the attachment of parts of wind turbines or of the protecting cover of nuclear power reactor shells. A number of threaded studs are usually arranged in a circular row for attachment of such kind of large mechanical assemblies.
In order to facilitate the attachment operations, for example of the protecting cover to a nuclear reactor shell, a supporting ring assembly is provided, said assembly being moved from above on the cylindrical cover installed on the nuclear power shell. The complete set of attachment studs is supported by the supporting ring which is also provided with all necessary means for screwing and unscrewing the studs as well as tensioning them in a longitudinal direction before tightening the corresponding nuts. In order to fulfill those tasks, the supporting ring is usually equipped with at least one and preferably two robot units capable of being moved along the periphery of the supporting ring for screwing, unscrewing and tightening the nuts one by one within the nuclear power shell. The longitudinal traction force for pre-stressing each individual stud before tightening is produced by a plurality of hydraulic stud tensioners, each of which is mounted on the head of a respective stud.
All the studs around the cover of the nuclear power shell must be submitted to the same longitudinal traction before the corresponding tightening nuts can be rotated so as to secure the cover to the nuclear power shell. It is therefore necessary to carefully monitor the traction force applied to all individual studs so as to guarantee an identical tension, and of course to maintain such force within specific limits depending on the tensile strength of the material used. Such monitoring is preferably made by measuring the elongation of each stud with elongation sensors, each sensor being mounted on a respective stud. The individual elongation sensors are usually mounted manually on each stud, which is particularly cumbersome. The sensors are supplied with electrical energy by electrical connections linked to a central power station. The measurement signals are transmitted from each individual elongation sensor by wire connections. Therefore, the mounting operation as well as the dismounting operation of the multiplicity of elongation sensors is particularly long and difficult.
One aim of the present invention is to avoid those drawbacks and to simplify the operations of assembling and disassembling a plurality of elongation sensors on a plurality of rod like members or studs to be tensioned in a multiple stud tensioning machine.
Another aim of the present invention is to permit a simpler monitoring of the elongation of a plurality of studs during a pre-tensioning operation.
More generally, the present invention aims at providing quicker and easier assembling and disassembling of a plurality of elongation sensors. A further aim of the invention is to provide a safer and cheaper monitoring of the elongation of a plurality of studs, during attachment of two mechanical elements together, for example, a protecting cover to a nuclear reactor shell.